A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium and a method of manufacturing the medium, and particularly relates to a perpendicular magnetic recording medium used for various magnetic recording devices including an external storage device for a computer, and a method of manufacturing the medium.
B. Description of the Related Art
As a technique for achieving an increase in recording density in magnetic recording, a perpendicular magnetic recording method is attracting attention as an alternative to the usual longitudinal magnetic recording method. This is because, compared with the usual longitudinal magnetic recording method, the perpendicular magnetic recording method has advantages of high density, high thermal stability, and sufficient writing capability even for a recording medium having high coercive force, making it possible to exceed a limit of recording density of the longitudinal magnetic recording method.
In a perpendicular magnetic recording medium, since information is recorded with a direction of magnetization being perpendicular to a film plane of a magnetic recording layer, magnetization needs to be kept stably in the direction perpendicular to the film plane. Therefore, a magnetic recording layer used for the perpendicular magnetic recording medium is required to have a high perpendicular magnetic anisotropy coefficient (Ku value). The Ku value of a magnetic recording layer of a perpendicular magnetic recording medium being currently studied is approximately 1×106 erg/cm3 or more.
In magnetic grains having uniaxial magnetic anisotropy, magnitude of a magnetic field necessary for reversal of magnetization is called anisotropy field Hk, and Hk is generally expressed as Hk=2Ku/Ms in terms of saturation magnetization Ms, and the Ku value. Therefore, to induce reversal of magnetization, a magnetic field of Hk or more is required, and a value of the magnetic field is proportional to the Ku value. In a magnetic recording medium, when Hk is excessively high, reversal of magnetization becomes insufficient during writing by a magnetic head, and thus normal operation is disabled, therefore a moderate Hk value is needed.
In a magnetic recording medium being an aggregate of magnetic grains, an average magnetization reversal field, of which the value is called coercive force Hc, is determined by distribution of the Hk values and axes of easy magnetization of individual magnetic grains, and strength of magnetic interactions between the magnetic grains and the like. When the magnetic interactions between the magnetic grains are small, the Hc value approaches the Hk value.
Moreover, an energy barrier E necessary for magnetization reversal is expressed as E=kuV(1−H/Hk)2, where H is a magnetic field applied in a direction of the axis of easy magnetization, and V is volume of a grain. When the energy barrier E is not sufficiently high compared with thermal energy kBT (kB is Boltzmann constant, and T is the absolute temperature), magnetization is reversed due to an effect of the thermal energy. This is called thermal fluctuation (or thermal disturbance) of magnetization, and implies loss of information in the magnetic recording medium, therefore the kuV value, which determines the energy barrier E, needs to be kept relatively high. Even if thermal fluctuation of magnetization does not lead to loss of information, it surfaces as a medium noise called reverse magnetic domain noise due to partial reversal of a recorded bit.
Typically, KuV/kBT is used as an index of the thermal fluctuation. However, this is used in cases where an external magnetic field is not being applied. When a magnetic field H is being applied, the index is expressed as kuV(1−H/Hk)2/kBT using the energy barrier E.
Furthermore, to reduce medium noises and thus improve quality of a recorded information signal by improving the signal-to-noise ratio (SNR), it is necessary to reduce a value of activation grain size D=V/δ (here, δ is thickness of a magnetic recording layer), i.e., to reduce a unit of reversal of magnetization. When the unit of magnetization reversal is small, a minute recording bit can be properly written, and consequently SNR is improved. Therefore, many studies have been made to reduce the D value in the perpendicular magnetic recording medium. To reduce the D value, it is effective to reduce crystal grain size of the magnetic recording layer, and reduce the magnetic interactions between crystal grains.
From the above, when the D value is reduced to improve SNR, the V value is reduced, therefore a high ku value is needed to keep a value of the energy barrier E necessary for stably keeping magnetization. On the other hand, when the ku value is kept high, the Hk value is increased, i.e., a magnetic field necessary for magnetization reversal is increased. This leads to difficulty in writing of information by a magnetic head. Thus, in the magnetic recording medium, it is extremely difficult to satisfy all three factors of (1) improvement in SNR, (2) thermal stabilization of magnetization (decrease in reverse magnetic domain noise), and (3) ease of writing by the magnetic head, which are in a tradeoff relationship with respect to one another.
As a perpendicular magnetic recording medium aiming to compatibly achieve improvement in SNR and thermal stabilization of magnetization among the three factors, a perpendicular magnetic recording medium has been proposed. The medium has a magnetic recording layer of a so-called functionally separated type in which a plurality of magnetic recording layers having different Ku values are stacked (see, for example, JP-A-11-296833 (U.S. Pat. No. 6,183,893 as U.S. counterpart patent), and JP-A-2000-76636 (U.S. Pat. No. 6,426,157 as U.S. counterpart patent).
In JP-A-11-296833 (U.S. Pat. No. 6,183,893 as U.S. counterpart patent), it is disclosed that a medium which is high with respect to thermal stability of magnetization and excellent in SNR can be produced by stacking a layer of a region having a high Ku value so that thermal stability of magnetization is high (upper layer), and a layer of a region having a somewhat low Ku value so that magnetic interactions between crystal grains are small and therefore SNR is high (lower layer). In one aspect, it is disclosed that the Ku value of the upper layer is adjusted to be 2.5×106 erg/cm3 to 5×106 erg/cm3, and the Ku value of the lower layer is adjusted to be 1×106 erg/cm3 to 2.5×106 erg/cm3. In JP-A-2000-76636 (U.S. Pat. No. 6,426,157 as U.S. counterpart patent), a similar technical idea is disclosed. Thus, it is disclosed that similar effects are obtained by stacking magnetic recording layers having different Ku values and crystal orientations. However, while JP-A-11-296833 (U.S. Pat. No. 6,183,893 as U.S. counterpart patent) and JP-A-2000-76636 (U.S. Pat. No. 6,426,157 as U.S. counterpart patent) disclose compatibly achieving improvement in SNR and thermal stabilization of magnetization, they make no consideration on ease of writing by the magnetic head.
With increase in recording density, the demand for reducing the D value and keeping the Ku value high in a medium has increased more and more, in order to stably keep a small recording bit. In such a medium, it is extremely important to secure ease of writing by the magnetic head.
In view of the above, the inventors applied an invention described in JP-A-2005-222675 (US2005/0181237A1 as US counterpart laid-open patent) aiming to provide a perpendicular magnetic recording medium in which thermal stabilization of magnetization and ease of writing by the magnetic head were compatibly achieved, and SNR also was improved. A summary of the invention described in JP-A-2005-222675 (US2005/0181237A1 as US counterpart laid-open patent) is as follows. That is, a perpendicular magnetic recording medium includes at least a nonmagnetic underlayer, a magnetic recording layer, and a protective layer formed in this order on a nonmagnetic substance, wherein the magnetic recording layer includes a low Ku layer having a perpendicular magnetic anisotropy constant (Ku value) of 1×105 erg/cm3 or less, and a high Ku layer having the Ku value of 1×106 erg/cm3 or more, i.e., the high and low Ku values differ at least ten fold. According to such a configuration, thermal stabilization of magnetization and ease of writing by the magnetic head can be compatibly achieved. Such operation is described as follows. In the magnetic recording layer in a two-layer structure including the low Ku layer and the high Ku layer, it is assumed that magnetic coupling of magnetization occurs in a thickness direction, and simultaneous magnetization reversal is thus induced. When a Ku value of the low Ku layer is ignored as an approximation, the total Ku value of the stacked films is decreased in accordance with increase in thickness, however, since the Ms value is kept even in the low Ku region, the total Ms value of the films is not significantly changed. Consequently, as is clear from the above relationship Hk=2Ku/Ms that the Hk value is effectively decreased, and reversal of magnetization is easily induced.
On the other hand, when the energy barrier E=kuV(1−H/Hk)2 is taken into consideration, since the V value can be regarded as the total volume of the films, the total KuV value of the stacked films is larger than a KuV value in the case of only the high Ku region. Here, since the Hk value is decreased as above, the decrease in energy barrier can be controlled at a small level, provided the externally applied magnetic field H is relatively low. That is, a medium is easily produced in which thermal stabilization of magnetization and ease of writing by the magnetic head are compatibly achieved.
According to the invention described in JP-A-2005-222675 (US2005/0181237A1 as US counterpart laid-open patent), a perpendicular magnetic recording medium can be provided in which thermal stabilization of magnetization and ease of writing by the magnetic head, particularly an overwrite characteristic described later, are improved, and furthermore SNR is improved. Overwrite is writing a new signal over an originally recorded signal without erasing the originally recorded signal. In a magnetic recording device, when data are overwritten, if the original data are not fully replaced by new data, an error may occur. The overwrite characteristic generally means a ratio of decay of an original signal when the original signal is overwritten with a subsequent signal, which represents overwrite performance.
Among patent documents, JP-A-2003-132532 and JP-A-2003-178416 are described while being compared with an embodiment of the invention in the section of Summary of the Invention.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.